Dictionarium polygraphicum. Burning of colours.

Dictionarium Polygraphicum:
Or, The Whole Body of Arts Regularly Digested.
Vol I.
London: Printed for C. Hitch and C. Davis in Pater-noster Row, and S. Austen in St. Paul's Church Yard. MDCCXXXV.
1735
There are several colours that require burning; as first lamp black, which is a colour of so greasy a nature, that except it is burnt, it will require a long time to dry.

The method of burning, or rather drying lamp-black, is as follows: put it into an iron ladle or a crucible, and set it over a clear fire, letting it remain till it be red hot, or so near it, that there is no manner of smoke arises from it.

Secondly, umber, which if it be intended for colour for an horse, or to be a shadow for gold, then burning fits it for that purpose.

In order to burn umber, you must put it into the naked fire in large lumps, and not take it out till it is thoroughly red hot; if you have a mind to be more curious, you may put it in a crucible, and then put it into the fire, till it be red hot; then take it out, and when it is cold, lay it up for use.

Ivory also must be burnt to make a black, thus: fill two crucibles with shavings of ivory; then clap their two mouths together, and bind them fast with an iron wire, and lute the joints close with clay, salt, and horse-dung well beaten together; then set it in a fire, covering it all over with coals, and let it re main in the fire, till you are sure the matter inclosed in the crucibles is thoroughly red hot; then take it out of the fire, but do not open the crucibles, till they are perfectly cold; for if you should open them while hot, the matter would turn to ashes; and so it will be, if the joints are not luted close; for it is only the exclusion of all air, that prevents any matter whatever that's burnt to a coal, from turning to a white ash, and preserves the blackness.